The object of the invention relates to a plunger lift pipet which comprises adjusting means for changing the travel of the plunger, display means for the liquid volume pipetted each time, disengageable coupling means arranged between the adjusting means and the display means for changing a pipet correction factor which determines the relation between the travel of the plunger and the indicated liquid volume.
Plunger lift pipets of the type mentioned at the beginning comprise a cone where. a pipet tip can be attached to. The attached pipet tip communicates with a cylinder where a plunger is arranged which can be actuated from outside. The plunger travel can be changed by means of adjusting means, in which case display means are provided for indicating each of the pipetted liquid volume. In most cases, however, the liquid volume absorbed by the pipet tip is different from the indicated one. The difference between the actually absorbed and the indicated liquid volume is also designated as a deviation in accuracy. Usually, the deviation in accuracy can be changed via the adjustment area of the plunger lift pipet.
Decisive for the degree of the deviation in accuracy and the results obtained via the adjustment area are the progress of the cross-section area along the plunger tip, the stagnant volume between the intake of the pipet tip and the plunger as well as the transmission parameters of the adjustment means for the plunger travel and the display means for the liquid volume. The dependence on the tip section and the stagnant volume results from the fact that the air cushion provided between the liquid level and the plunger is weighted by the liquid column in the tip, for which reason the air cushion is increased in size.
A system consisting of a plunger lift pipet and a pipet tip shows a defined deviation in accuracy for any certain liquid at any adjusted liquid volume (compare also DE 42 09 620 C1). If, however, the liquid density changes with the liquid to be pipetted, a change of the weight by which the liquid column weighs down the air cushion and of the deviation in accuracy of each adjusted liquid volume results as well.
An adaptation of the deviation in accuracy may be performed by changing a pipet correction factor. It is decisive for the relation between the adjusted plunger travel and the adjusted liquid volume. A few known plunger lift pipets have already been provided with an adjusting means for the pipet correction factor.
One possibility is the adjustment of a sliding clutch between the adjusting means for the plunger travel and a counter mechanism. For this purpose, the pipet body comprises an aperture through which a screw driver can engage a sliding clutch of a bevel gear set. The connection between the volume adjustment and the counter mechanism can be changed by holding the volume adjusting means and turning the screw driver or vice versa. A control of the deviation in accuracy so obtained may be made by weighing a pipetted quantity. This adjusting means of the pipet correction factor is not particularly easy accessible from outside and, in most cases, is only used for a factory adjustment or, respectively, by the customer in special cases.
As concerns the adjustable dilution means according to the DE-A1-27 20 669, the adjusting means comprise an adjustable screw nut for adjusting an abutment for the plunger dipping means, said abutment being tightly connected to the adjustable screw nut and said adjustable screw nut being connected to a threaded rod via a cornet screw which threaded rod is connected to a volume display. The cornet screw can be removed for adjusting the zero setting of the volume display and the abutment can be changed by means of the adjustable screw nut.
The U.S. Pat. No. 4,165,646 discloses a pipetting means, the adjusting means of which comprise an actuating bushing for receiving the upper end of an abutment bushing of a plunger rod. Thus, the plunger travel can be changed by turning the actuating bushing. Said actuating bushing at the same time includes a covering bushing which surrounds the abutment bushing and the upper portion of a cylindrical body. The body includes a volume scale and the covering bushing comprises a window for making all adjusted volume value visible. According to a modification of this means, the covering bushiug is adjustably secured to the upper bushing for calibration purposes.
The U.S. Pat. No. 4,501,163 describes a pipet means which includes a head for an axial plunger displacement and for adjusting the plunger travel by turning an abutment bushing which head, at the same time, is connected to a counting mechanism so as to be secured against rotation. This means can be calibrated by turning a bushing which receives the abutment bushing for the plunger rod by an internal thread.
The U.S. Pat. No. 4,023,716 discloses a micropipet which in an axially displaceable plunger rod comprises a displacement plunger projecting from the lower end of said plunger rod. The plunger is secured within the rod by means of a screw and can be axially displaced after removing said screw and secured to another position then. In this way, an adjustment of the liquid quaunity to be measured out can be realized.
The EP-A1-0 286 676 describes a pipet allowing the volume to be measured out to be changed very quickly. According to this means, all adjusting bushing is permanently connected to a display means for adjusting the plunger travel. The position of the bushing is changeable by means of an adjusting nut. The nut, however, call be disengaged from its toothing with the bushing which causes the bushing to be adjustable for the travel of the plunger with the aid of a control knob. This favours a quick adjustment of very different quantities of the volumes to be measured out.
Taking all this into cousideration, it is the object of the invention to provide a plunger lift pipet featuring a simple and safe adjustability of the deviation in accuracy.